1. Field of the Invention
The invention relates to a flexible optical waveguide and a process for its production.
2. Description of the Related Art
Along with the practical applications of optical transmission systems, techniques relevant to optical waveguides as their basic components have drawn much attention. An optical waveguide has, typically, an embedded type structure in which a core layer having a high refractive index is surrounded with a cladding layer having a low refractive index, or a ridge type structure in which a core layer having a high refractive index is formed on a lower cladding layer having a low refractive index and an upper cladding layer is an air layer. Thus, light incoming to the optical waveguide is transmitted in the core layer while being reflected at the interface between the core layer and the cladding layers or at the interface between the core layer and the air layer.
Recently, there have been developed opto-electronic hybrid integrated modules each comprising an optical waveguide and an electronic circuit disposed on a single substrate. For example, Japanese Patent Laid-open Publication (Kokai) No. 2001-15889 discloses an opto-electronic wiring board obtained by attaching an optical waveguide film to a multilayer wiring board with an adhesive. Further, Japanese Patent Laid-open Publication (Kokai) No. 2002-189137 discloses an optical wiring board obtained by attaching an optical waveguide part, which is formed on a transparent substrate, to an electronic circuit board with an adhesive. Further, Japanese Patent Laid-open Publication (Kokai) No. 2004-341454 discloses an opto-electronic hybrid integrated substrate obtained by attaching an optical waveguide film to an electronic circuit board with an adhesive.
However, such opto-electronic hybrid integrated modules, each of which is obtained by attaching an optical waveguide film to an electronic circuit board with an adhesive, have a problem that the electronic circuit board and the optical waveguide film are easily separated from each other at the time of a wet heat test. Further, in order to lead light emitted from a light emitting element mounted on an electronic circuit board to an optical waveguide, the light needs to pass through an adhesive layer, at which time light scattering is caused because of a mismatch in refractive index between the optical waveguide film and the adhesive layer, and therefore, there is a problem that the waveguide loss of the optical waveguide becomes high. Further, even if an opto-electronic hybrid integrated module has flexibility to a certain extent, in the case where an adhesive layer exists, there is also a problem that the module is weak in bending, and therefore, the electronic circuit board and the optical waveguide film are easily separated from each other at the time of a bending test.
Thus, Japanese Patent Laid-open Publication (Kokai) No. 2006-22317 discloses an opto-electronic hybrid integrated flexible module obtained by previously producing epoxy resin films to be a lower cladding layer, a core layer, and an upper cladding layer of an optical waveguide, successively vacuum laminating these epoxy resin films onto a copper-clad polyimide substrate, and then curing the resulting films for directly forming an optical waveguide film on an electron circuit board without using an adhesive.